Individuals infected with HIV-1 display a wide range in rates of disease progression. Rare individuals show no evidence of disease even after more than ten years of infection. These long term nonprogressors (LTNP) generally exhibit extremely low levels of HIV-1 in the circulation and maintain normal numbers of CD4 T-cells. Our studies are intended to examine individual cases of long-term non-progressive HIV-1 infection in great detail. We propose to investigate the hypothesis that many LTNP are infected with attenuated viral strains with identifiable, uniform and persistent genetic defects. Subjects will include asymptomatic, untreated adults and adolescents who have had stable and normal CD4 T-cell counts for more than 10 years of HIV-1 infection and vertically infected children with normal CD4 T-cell counts after 5 or more years of infection. Virological studies will include measurement of viral load, and phenotypic characterization of primary viral isolates with assays of tropism, cytopathogenicity and replication kinetics. Long distance amplification of proviral DNA will be undertaken to generate infectious clones directly from cultured virus of non-progressors to use in full length sequence analyses. Primary isolates which replicate poorly will be analyzed to determine the genetic basis of the defects. For those LTNP from whom virus cannot be isolated, targeted sequence analyses and functional studies will be performed on accessory and envelope genes. The effects of accessory gene sequence defects, if uniformly present, will be further characterized in functional assays. Envelope sequences will be studied with chimeric reconstructions to measure properties of tropism, infectivity and cytopathogenicity. These studies will provide fundamental insight into the relative importance and functional roles of individual genes and gene regions in the natural setting of human HIV-1 infection. Overall, this information will be useful for the further development of attenuated HIV-l strains for vaccine trials.